1. Field of Invention
This invention relates to the use of vapor phase epitaxy to grow a crystal layer on a surface, specifically to the growth of a nitride semiconductor crystal ingot in a hydride vapor phase epitaxy reactor.
2. Discussion of Prior Art
Epitaxy is the growth of a mono-crystalline layer on a mono-crystalline substrate. Vapor phase epitaxy (VPE) achieves this by reacting one or more source gases, or “precursors” on the substrate.
Hydride vapor phase epitaxy (HVPE) is a form of VPE in which the vapor-phase precursors typically comprise a halide of a group III metal (IUPAC group 13), and a hydride of a group V element (IUPAC group 15) resulting in a “III-V” material. HVPE has been widely used in fabricating an important subgroup of the materials called the “III-nitrides.” The III-nitride materials include gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN) and combinations formed from these three that may be doped with additional elements to customize their band-gap or lattice parameters. III-nitride semiconductors are used as substrates and as component layers in high-brightness light emitting diodes (HB-LEDs) and blue laser diodes (LDs). HB-LEDs are expected to replace compact fluorescent lights and traditional incandescent lighting over the next few years. The III-nitride substrate also has great advantages for a variety of high-power, high-frequency, and high-temperature integrated circuit applications. The ongoing problem is that the III-nitride substrates are very expensive and difficult to produce.
In epitaxy, the growing crystal layer indexes its structure to the crystal structure of the substrate. If the unit cells of the growing crystal layer have length or width dimension that differ from those of the substrate crystal, they may deform in those dimensions to force a match between crystal lattices at the layer-substrate interface. The resulting deformation accumulates in the plane of the interface until a crystal growth dislocation, or “defect,” is generated to relieve some of the strain in the layer. Such defects are replicated in subsequent layers, propagating from the substrate interface upward through the many layers of the grown, crystal, until the crystal accumulates enough volume to absorb the strain, at which point the defect is sometimes absorbed or grown over. Similar defects may also form as a result of, variations in the growth conditions across the substrate, in which case they are continually generated during growth.
A III-nitride substrate is cut from a thick single-crystal ingot. Ideally, it is cut from an area where there are fewer defects, far enough from the substrate interface so that the defects from lattice mismatch are at a minimum. Unfortunately, thick III-nitride ingots have proven to be very difficult to grow. The internal stress and defects, induced during initial ingot growth on lattice-mismatched substrates and further generated by non-uniform reactor growth conditions, have limited practical ingot width to about 50 mm (2″). Furthermore, the low material efficiency inherent to the prior art HVPE technology has added to their high cost.
In conventional HVPE, flowing halide and hydride source vapors, or “precursors,” are heated and mixed upstream from a substrate, then flowed downstream to cross the substrate and finally exit through an exhaust port. As the precursor mixture flows across the substrate, the precursors react on the surface to form the III-nitride crystal layer. At the same time, this reaction releases hydrogen (H2) and hydrogen chloride (HCl). In the case of GaN HVPE, used here as illustrative example, the surface reaction is described by the following relation:NH3(gas)+GaCl(gas)→GaN(solid)+HCl(gas)+H2(gas)  Eq. 1
Hydrogen chloride and H2 gas evolved in the reaction are entrained in the gas stream as it crosses the surface. Within the flow-induced boundary layer, which forms between the growth surface and the main gas stream, the gas composition is continuously depleted of NH3 and GaCl, and enriched with the reaction byproduct gases, HCl and H2. Additionally, this boundary layer goes through a transition from laminar to turbulent flow and gets thicker with distance. The changing structure and composition of the flowing boundary layer generates a non-uniformity of growth conditions across the surface. This non-uniformity of growth conditions leads to a non-uniformity of growth kinetics and the accumulation of internal strain, which contributes to defect generation and reduced crystal stability.
In FIG. 1A, the effects described above are illustrated. A gas stream flowing from the left to the right across a surface at first exhibits laminar flow, but when the boundary layer reaches a critical thickness, drag begins to force the gas stream to rotate near the substrate, and flow becomes turbulent. The overall flow continues rightward, while the boundary layer gets thicker and accumulates more HCl and H2.
In FIG. 1B, the effect of no lateral gas flow is illustrated. As the surface reaction of Eq. 1 proceeds, the thin layer of gas mixture near the growth surface become depleted of NH3 and GaCl, and enriched with the reaction byproduct gases, HCl and H2. This causes the layer to become buoyant, and it breaks up into small volumes of gases that move upward. These rising “micro-plumes” are replaced by downward falling plugs of heavier gas that is still rich with NH3 and GaCl. The resulting turbulence averages out within a short lateral distance, and results in highly uniform average conditions across the substrate.
Current HVPE reactor technology is classified according to the way precursors are flowed in relation to the substrate. Horizontal flow reactors flow the gas mixture parallel to and across the substrate surface, while vertical reactors flow the gas mixture at 90 degrees to the substrate surface, forcing flow to be diverted across and around the substrate.
Examples of horizontal flow reactors are described by G. M. Jacob and J. P. Hallais, and by T. Shibata et al. In horizontal flow HVPE reactors, the substrate is oriented approximately parallel to the gas stream, and may be rotated to help even out the effect of gas stream composition, changes. Nonetheless, surface elements at the center of the substrate do not experience the same process environment as surface elements radially displaced toward the edge, the H2 and HCl content increases toward the center, and this effect gets more pronounced as the diameter of the substrate is increased. This issue grows as substrate size increases, since the boundary layer grows thicker as it flows further across a substrate. The changing concentration of HCl and H2 in the flow-induced boundary layer causes a variation in growth conditions across the substrate, and results in lateral internal strain within the growing layer. Increasing substrate size increases the distance over which these effects accumulate, and conventional HVPE has therefore been unsuccessful in scaling GaN growth process wider than about 50 or 60 mm (2-2.5″).
In the continuous flow of precursor gas past a substrate, only a small portion is reacted at the surface, while the rest is swept downstream unused. In the case of horizontal flow GaN HVPE, about 95% of the GaCl passes through the reactor without contributing to crystal growth at the substrate. Some of the unused precursor gas mixture forms “parasitic deposits” of polycrystalline GaN on the reactor walls, the rest exits through an exhaust duct and must be treated and have its gallium reclaimed. This inefficiency is expensive, and contributes to the high cost of even small GaN substrates.
An example of a vertical flow HVPE reactor is described by G. B. Stringfellow and H. T. Hall. Vertical flow HVPE reactors position the substrate facing the gas flow, causing variations in pressure and velocity as the gas mixture is redirected around the substrate. In vertical flow, the boundary layer is compressed and the H2 and HCl content increases toward the periphery. A compressed boundary layer is a lower barrier to diffusion mass transport, and the vertical flow orientation intersects more of the gas stream. These two factors help vertical flow HVPE reactors to operate at better efficiency than horizontal flow versions, getting 5-12% gallium utilization, compared to 2-7% in horizontal flow designs. As with the horizontal flow, however, radial difference in growth conditions lead to ingots with high internal strain and defects, limiting the size and quality of crystals that can be grown to about 50 mm (2″).
Both horizontal and vertical HVPE reactors are considered “open” designs; gases flow in at one end and out at the other. A “closed” reactor re-circulates reactants within a closed container, and so has the potential to be extremely efficient. In the case of GaN growth, closed reactor designs are typically not vapor-phase reactors, but liquid-phase ammonothermal designs. In ammonothermal growth, an autoclave is used to contain the high pressure and temperature needed for liquid-phase GaN growth, and crystals grown this way are reported to have very low defect densities. The growth rate is slow, however, and the crystals are small, irregularly shaped, and often contain high levels of unintended dopants. An, example of ammonothermal GaN growth is taught by M. P. D'evelyn et al.
A closed HVPE reactor design is disclosed by Jai-yong Han. Han claims that gas flow to the substrate is driven by thermal convection, since he maintains the lower portion of his reactor at a higher temperature than the upper portion: hot gases rise up the walls, cool and become denser in the upper portion, and descend down the central axis past a downward-facing substrate. Han reports growth rates roughly half of conventional. HVPE, but claims the efficiency of his reactor is about 20-25%, with 75-80% of the gallium being consumed in parasitic depositions. Still, this is twice the efficiency of even the most efficient vertical open flow reactor designs.
Like most conventional HVPE reactors, Han's contains a receptacle for holding liquid gallium (Ga) metal. HCl gas reacts with the Ga surface and creates the GaCl precursor according to the following equation:HCl(gas)+Ga(liquid)→GaCl(gas)+½H2(gas)  Eq. 2This is the source of GaCl needed for the growth reaction described by Eq. 1. The initial production of GaCl within a closed HVPE reactor is “primed” by the injection of HCl, and then sustained by the HCl released and recycled during GaN growth. Recycling HCl in this application involves the consumption of NH3, per Eq. 1, and Eq. 2 indicates that an overpressure of H2 is continuously created by the HCl cycle. The increasing H2 content within Han's closed reactor changes the balance of Eq.s 1 and 2 and also leads to continuously increasing gas pressure.
In a closed HVPE reactor where no material is added or released, all reactions stop when either the hydride (i.e., NH3) or the metal (i.e., Ga) are used up. During this time, the reactor's internal pressure will have increased 25% due to the evolution of H2 gas. Further input of NH3 will restart the growth reactions, but the addition of one unit of NH3 will result in a pressure increase proportional not only to the one unit, but to another half unit from H2 production.
If Han's closed reactor is operated at 10 atmospheres of pressure, the upper limit cited by Han, then the enclosed gas volume at 800° C. must be 6100 times the finished ingot volume (at 25% efficiency, as defined by Han). For example, to grow an ingot 50 mm (2″) in diameter and 10 min (⅜″) thick, the reactor must be roughly 60 cm (24″) in diameter and 60 cm (24″) tall. To grow an ingot that would yield about 40 wafer substrates 100 mm (4″) in diameter would require a reactor 1.2 meters (4′) tall and 1.2 meters (4′) wide. Considering the temperature and pressure requirements, this would be a very expensive and potentially dangerous quartz vessel. Scaling it to grow ingots for state-of-the-art production (for example, 80 wafers at 200 mm diameter) would require a chamber over 4 meters (12′) tall and 2.2 meters (7′) in diameter, heated to 800° C. and containing 10 atmospheres (140 p.s.i.) of pressure. Unless large and inexpensive hyperbaric reactor vessels become available, closed reactor HVPE is not going to be an economically practical solution.
It has been assumed that nitrogen gas (N2) is generated within a closed or open GaN HVPE reactor, though in smaller volumes than H2—Several potential NH3 and N2-producing reactions have been proposed in the literature, including the following:GaN(solid)+HCl(gas)→GaCl(gas)+½H2(gas)+½N2(gas)  Eq. 32NH3(gas)→N2(gas)+3H2(gas)  Eq. 42GaN(solid)+3H2(gas)→Ga(liquid)+2NH3(gas)  Eq. 5GaN(solid)+HCl(gas)+H2(gas)→GaCl(gas)+NH3(gas)  Eq. 6
Han sees no parasitic GaN growth in the hotter (lower) part of his reactor, asserting that the reaction described in Eq. 3, above, dominates at higher temperatures, and prevents it.
Han's assertion is not well-supported by the reports of others, however, and he does not report measuring any N2 in his reactor. In fact, no reports of N2 generation in actual GaN HVPE environments have been found. This inconsistency may lie in the assumption that reactions such as those described in Eq. 3, 4, and 5, though valid as stand-alone reactions, are unaffected by the presence of other HVPE gases. For instance, M. A. Mastro et al. confirmed that HCl alone (Eq. 3) would etch a GaN sample, and M. Furtado and G. Jacob showed that a mixture of HCl+H2 (Eq. 6) will even more aggressively etch a GaN sample, but A. Trassoudaine et al. showed that the etching of GaN by HCl alone (Eq. 3) did not occur in an HVPE environment where NH3 was present.
On the contrary, Trassoudaine teaches that additional HCl could enhance epitaxial GaN growth rates on the (001) plane while reducing parasitic GaN growth on reactor walls in. The enhanced (001) growth was attributed by Trassoudaine to a de-chlorination mechanism, while the reduction in parasitic GaN growth with excess HCl may have more to do with the exposure of highly reactive non-(001) GaN crystal planes exposed in polycrystalline GaN (i.e., parasitic deposits) vowing on non-epitaxial reactor walls. This theory is supported by Mastro, who reports variations in GaN crystal plane reactivity, especially in the presence of HCl. Furthermore, Furtado's work indicates that the reduction of parasitic GaN growth is most correctly explained by HCl+H2, not HCl alone, and results in the production of GaCl+NH3, as in Eq. 6, which is just the reverse of Eq. 1. Thus, the production of N2 in a GaN HVPE environment is undetected and unlikely, and the only significant non-recyclable gas byproduct of GaN HVPE is H2.
Han's closed reactor reportedly grows a very high quality, low defect GaN ingot without flowing precursors, and infers that this is due to the use of GaN substrates. Han claims a convection flow driven by the temperature difference between the lower and upper portions of his reactor, the lower portion being hot enough to suppress parasitic nucleation and the upper portion maintained 100° C. to 200° C. cooler. This infers that GaN growth will occur at the substrate and on all surfaces within the upper, cooler portion.
What is needed is 1) an HVPE reactor that can eliminate the non-uniform effect of a flow-induced boundary layer so that large III-V crystals can be grown, and 2) a way to conserve and recycle precursor gases within the HVPE reactor and grow large ingots without requiring large, expensive reactors.